"Cecil Moore" wrote in message
Richard Fry wrote:

"Cecil Moore" wrote
Passive non-resonant elements have very little effect.
That's what allows a lot of multi-band beams to work.

As a point of reference, the radiation patterns of sidemounted FM
broadcast transmit arrays are affected by a non-resonant mounting
structure (the tower) -- especially for vertical polarization.

True, and please note that I was talking about *antenna
elements* not support structures.
___________________

But geing closer to the driven element(s), probably even non-resonant
antenna elements may have more affect on the net antenna pattern than that
produced by a supporting tower further away.

RF

Richard Fry wrote:
But geing closer to the driven element(s), probably even non-resonant
antenna elements may have more affect on the net antenna pattern than
that produced by a supporting tower further away.

Using EZNEC, one can observe that the current in a
closer non-resonant passive antenna element can be
pretty low while the current in a grounded tower
further away can be pretty high.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil, W5DXP wrote:
"Passive non-resonant elements have very little effect."

True in a parasitic array such as a Yagi.

In some antenna arrays all the elements are fed by a transmission line.
Directional arrays are also made by using elements which are not
connected to a transmission line. These disconnected elements have
current induced into them by the driven element and are called parasitic
elements.

The effect of a parasitic element depends on the magnitude and phase of
the current flowing in it. To get substantial current, the element must
be near the driven element or other element carrying large current, and
it must be almost resonant. A non-resonant element has a large reactance
limiting its current flow and effectiveness.

A resonant length for a conductor in space is 1/2-wavelength.

A resonant length for a grounded conductor is 1/4-wavelength.

The 3-element Yagi is a common parasitic antenna. It has a driven
element, a reflector element, and a director element. The length of all
three elements is nearly 1/2-wavelength in most cases. To get large
currents, near-resonance is essential.

The trick is to get the radiation from all elements to reinforce in the
desired direction and cancel in other directions.

Phase of current in the reflector is often delayed by making it slightly
longer than a 1/2-wavelength. Phase of current in the director is often
slightly advanced by making it slightly shorter than a 1/2-wavelength.

Spacing between elements is close to get a large excitation of the
parasitic elements. This makes a compact antenna. Close spacing means
mutual impedance (coupling) between the elements, and this affects the
drivepoint impedance. Close spacing increases directive gain but lowers
the array`s radiation resistance. See Terman`s Fig 23-36 on page 904 of
his 1955 "Electronic and Radio Engineering".

Large non-resonant reflectors are also used. Backward radiation can be
eliminated with a plane conducting sheet. It`s impenetrable and must
reflect. The earth often acts as such a reflector. The parasitic
1/2-wave reflector is a "degenerated" (Kraus` word) case of the plane
reflector. I recall that either Kraus or Terman suggested that the best
way to handle non-resonant reflectors was on an image basis.

Best regards, Richard Harrison, KB5WZI

I believe that you could probably get a fairly effective
pattern-warping by using a non-tuned reflector which is significantly
longer than your collinear radiator.

Passive non-resonant elements have very little effect.
That's what allows a lot of multi-band beams to work.

As a point of reference, the radiation patterns of sidemounted FM broadcast
transmit arrays are affected by a non-resonant mounting structure (the
tower) -- especially for vertical polarization. Measured patterns from the
manufacturer's test ranges demonstrate this, as do the NEC-2 studies in
several of the papers on http://rfry.org .

Check... that's just the sort of situation I was thinking of.

The effect on the pattern isn't all that strong, though... only a few
dB. As one example, the old ARRL VHF book gives the plans for a
four-bay stacked dipole antenna system. With the dipoles placed on
four sides of the mast, the antenna system has a gain of about 6 dB
(they don't say dBi or dBd but I assume it must be the latter). With
all four dipoles on the same side of the mast, the pattern is said to
be a cardioid of about 9 dB gain.

They don't say how deep the back-side null is. The cardioid pattern
might have a high enough F/B ratio to help with the original poster's
situation (mountain-side reflection), or he might need to use a tuned
reflector of the 5%-longer-than-resonant variety to get a more
directional pattern.

--
Dave Platt AE6EO
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